Tasmanian teacher and mum who raped three students says, "I thought 16 was the legal age."

A Tasmanian high school teacher who had sex with three of her under age students will never set foot in a classroom again, instead she’ll spend at least the next eight months behind bars.

Recently qualified Casey Lee Sullivan, 33, would send “flirtatious” texts and Facebook messages to the boys, aged 15 to 16, and had sex with them in her car, at the beach and on school property.

“I actually thought 16 was the legal age and I found out later it wasn’t,” she told police at the time of her arrest, according to the Hobart Mercury.

Sullivan met the first boy while she was still a student teacher in 2012.

The then-married mother-of-two said she had sex with him because he paid her attention.

The following year she began pursing another boy via Facebook, eventually picking him up from his parent’s house and taking him to a nearby beach for sex.

Last year she had a longer affair with a third student who was 15.

Sullivan contacted him on social media and took him to the school gym where they had sex on the gym mats.

He later told his friends about it, showing them the messages and saying another teacher had almost caught them in the act.

Sullivan had sex with a 15-year-old student multiple times on school property. Source: iStock

The pair had sex several more times, including in school corridors, and Sullivan would often send sexually explicit texts and photos.

Police were contacted when rumours began circulating and Sullivan confessed.

'The crimes were so serious, particularly in light of her position as a teacher ... that a significant prison sentence is called for," Justice Blow told the Tasmanian Supreme Court this week, Sky News reports.

"There are likely to be long-term consequences with his sexual development," he said, referring to the youngest of the students who was in Year 9.

Sullivan plead guilty to nine counts of intercourse with boys and was sentenced to 16 months in prison.

She'll be eligible for bail in eight, but is now on the sex offender registry for seven years and will never teach again.

The judge acknowledged her remorse and the impact a long imprisonment could have on her two kids.